Piezoelectric fuel injectors generally employ piezoelectric actuators comprised of a stack of piezoelectric elements, arranged mechanically in series, to open and close an injection valve in order to inject fuel into a combustion space. Typically, a piezoelectric actuator is located in a chamber, defined by an injector housing, which contains fuel at injection pressures. The piezoelectric actuator controls movement of the injection valve either by means of a mechanical or hydraulic coupling. An example of one such piezoelectric fuel injector is disclosed in U.S. Pat. No. 6,520,423.
A typical multi-layer piezoelectric actuator or stack is shown in FIG. 1 and comprises a stack body 12, which is generally rectangular in cross section, having first and second opposing outer faces 14, 16. The opposing faces 14, 16 of the stack body 12 are provided with first and second conductors or distribution electrodes 18, 20 respectively. The stack body 12 comprises a plurality of relatively thin piezoelectric ceramic layers or elements 22, each of which is spaced from adjacent elements 22 by an internal electrode 24. Alternate ones of the internal electrodes 24 are electrically connected to the first distribution electrode 18 and second distribution electrode 20, respectively, to form two groups of electrodes, whereby the electrodes of one group are interdigitated with the electrodes of the other group.
With reference to FIG. 2, a voltage is applied, in use, across the two distribution electrodes 18, 20 whereby an adjacent pair of internal electrodes 24 sandwiching a piezoelectric element 22 become mutual conductors of opposite polarity and apply an electric field to the intermediate element 22. Put another way, the distribution electrodes 18, 20 serve to distribute charge to each group of the internal electrodes 24 to which they are connected. When an electric field is applied to the distribution electrodes 18, 20, the piezoelectric actuator will elongate (if the piezoelectric actuator is of the energise-to-extend type) or contract (if the piezoelectric actuator is of the de-energise-to-extend type) along its longitudinal axis.
It will be appreciated that each piezoelectric element 22 is a dielectric material, such that it is a poor conductor of electrical charge and, therefore, serves to insulate opposing internal electrodes 24 whilst supporting electric fields generated therebetween. Typically, each piezoelectric element 22 has a thickness of around 100 μm, so a sufficient electric field strength of, for example, 2 kV/mm can be attained between the internal electrodes 24 by way of a relatively low applied voltage of 200V, whilst still obtaining the required elongation or contraction of the actuator stack. However, if the voltage across a piezoelectric element 22 becomes too great, that is to say if the electric field becomes too intense, the dielectric material will begin to breakdown and so will conduct electric current.
Dielectric breakdown of the piezoelectric elements in multilayer stack-type actuators severely affects actuator functionality. For instance, dielectric breakdown of just one piezoelectric element will cause a low resistance (short circuit) path between two opposing internal electrodes 24. Since the internal electrodes 24 are connected in parallel with the distribution electrodes 18, 20, the electric field in each element 22 will dissipate as charge is no longer distributed to the unaffected internal electrodes 24. Therefore, the piezoelectric actuator is rendered inoperative.
Dielectric breakdown also causes permanent structural damage to the piezoelectric actuator. This is due to the high current flowing through the short circuit path elevating the temperature of the surrounding ceramic material to such an extent as to cause melting of the ceramic elements 22 and the internal electrodes 24 of the piezoelectric actuator.